An Erbium Doped Fiber Amplifier (EDFA) is a fiber-based laser amplifier. The fiber is doped with Erbium ions (Er+), which have upper energy lifetimes on the order of 10 ms, for example. This, in addition to the wavelength of operation being pronounced from 1530 nm to 1570 nm, makes an EDFA ideally suited for long-haul fiber optic systems. The use of EDFAs has historically been applied most extensively to creating a strong transmitted signal, but it has also been used as a pre-amplifier for low signal level receivers. Their use as a low signal level pre-amplifier for optical communications has been limited by the noise figure of an EDFA. Due to the “on-off” nature of the optical signal level during optical communications, there are times when there is not (and should not be) any significant signal in the fiber. During these times, even though they may be extremely short duration, the Erbium ions remain in an excited state and are free to spontaneously emit at any time. Although there is high probability that a spontaneous emission will not occur during such a short period of time, any photon that is spontaneously emitted is amplified by the fiber, leading to significant noise.
A technique is needed to reduce the noise associated with spontaneous emissions in a fiber amplifier device.